Perpendicularly magnetized MTJs (p-MTJs) are a major emerging technology for use in embedded magnetic random access memory (MRAM) applications, and standalone MRAM applications. P-MTJ MRAM technology that uses spin-torque (STT-MRAM) for writing of memory bits was described by J. Slonczewski in “Current driven excitation of magnetic multilayers”, J. Magn. Magn. Mater. V 159, L1-L7 (1996), and is an increasingly promising candidate for future generations of non-volatile memory to replace embedded flash memory and embedded cache memory (SRAM).
Both MRAM and STT-MRAM have a p-MTJ cell based on a tunneling magnetoresistance (TMR) effect wherein a stack of layers has a configuration in which two ferromagnetic layers are separated by a thin insulating tunnel barrier layer such as MgO. One of the ferromagnetic layers called the pinned layer has a magnetic moment that is fixed in an out-of-plane direction such as the +z direction when the plane of each layer is laid out in the x-axis and y-axis directions. The second ferromagnetic layer has an out-of-plane magnetization direction that is free to rotate to either the +z-axis (parallel or P state) or the −z-axis (antiparallel or AP state) direction. The difference in resistance between the P state (Rp) and AP state (Rap) is characterized by the equation (Rap−Rp)/Rp also known as DRR or the MR ratio. It is important for p-MTJ cells to have a large MR ratio, preferably higher than 100%, as the MR ratio is directly related to the read margin for the memory bit, or how easy it is to differentiate between the P state and AP state (0 or 1 bits).
Another critical requirement for p-MTJs is thermal stability to 400° C. that is a typical temperature during back-end-of-line (BEOL) processes when fabricating embedded memory devices in complementary metal-oxide-semiconductor (CMOS) products. A general trend has been to introduce a second metal oxide/free layer (FL) interface similar to the tunnel barrier/FL interface thereby enhancing PMA and Hk within the free layer, and improving thermal stability. Thermal stability (Δ) is a function of the perpendicular anisotropy field as shown in equation (1):
                    Δ        =                                            M              S                        ⁢                          VH                                                k                  eff                                ,                ⊥                                                          2            ⁢                          k              B                        ⁢            T                                              (        1        )            where Ms is the magnetic saturation value, Hkeff,⊥ is the out-of-plane (perpendicular) anisotropy field, V is the volume of the free layer, and where kB is the Boltzmann constant, and T is the temperature.
The perpendicular anisotropy field (PMA) of the free layer is expressed in equation (2) as:
                              H                                    k              eff                        ,            ⊥                          =                                            -              4                        ⁢            π            ⁢                                                  ⁢                          M              s                                +                                    2              ⁢                              K                U                                  ⊥                                      ,                    s                                                                                                      M                s                            ⁢              d                                +                      H                          k              ,              𝒳              ,              ⊥                                                          (        2        )            where d is the thickness of the free layer, Hk,χ,⊥ is the crystalline anisotropy field in the perpendicular direction, and KU⊥,s is the surface perpendicular anisotropy of the top and bottom surfaces of the free layer. Thus, PMA is increased with the introduction of a second free layer/metal oxide interface, which enhances the surface (interfacial) perpendicular anisotropy component. Higher PMA is especially important to allow data retention at small device sizes.
Typically, a Fe rich alloy is used as the free layer, and MgO as the tunnel barrier and Hk enhancing layer to enable lattice matching between the layers, and the use of MgO as a spin filtering element, providing an optimum MR ratio and excellent read signal for the device. Moreover, boron is commonly included in the Fe rich alloy that is CoFeB, for example, to allow an amorphous free layer to be deposited that crystallizes during a subsequent anneal to promote lattice matching with the MgO tunnel barrier and Hk enhancing layer. Unfortunately, the presence of boron in the free layer leads to a lower moment (Ms) and reduced PMA. Although boron segregates to a certain extent from the magnetic element(s) in FeB or CoFeB during annealing, the desired Ms value of an as-deposited Fe or CoFe layer is never realized. Moreover, the lower Ms value of the boron containing free layer favors a non-uniform reversal mechanism of the free layer during switching from a P to AP state, or vice versa, which in turn lowers the energy barrier for switching and causes lower thermal stability.
An improved process for fabricating a free layer in a p-MTJ is needed so that the advantage of depositing an amorphous boron containing free layer for optimum lattice matching may be retained without leading to a lower Ms and compromising PMA and thermal stability after an anneal step is performed.